Technical Field
The present invention relates to an electron spin resonance apparatus (ESR apparatus), and in particular to a technique for detecting an ESR signal by lock-in demodulation.
Related Art
An electron spin resonance apparatus (ESR apparatus) is a type of a magnetic resonance apparatus, which irradiates a microwave onto a sample placed in a static magnetic field, and which records absorption of the microwave by the sample in the form of a spectrum.
FIG. 7 shows an example of an ESR apparatus. The ESR apparatus is an apparatus which can execute continuous wave (CW) ESR and pulse ESR. Some ESR apparatuses are capable of executing only the continuous wave ESR.
First, the continuous wave ESR will be explained. A sample tube with a sample 100 disposed inside is inserted into a microwave resonator 102. The microwave resonator 102 is placed between two electromagnets 104. With such a configuration, the microwave resonator 102 is placed in a static magnetic field generated by the electromagnets 104. In the continuous wave ESR measurement, a magnetic field modulation coil 106 is used. For example, a magnetic field modulation coil 106 is placed outside of the microwave resonator 102.
When the continuous wave ESR is executed, a path 116 is formed by switches 112 and 114. A microwave generated by a microwave oscillator 108 is attenuated by an attenuator 110 to a predetermined electric power, and is then supplied to the microwave resonator 102 through the path 116 and a circulator 118. After a degree of coupling between the microwave line path and the microwave resonator 102 is adjusted to a state where there is almost no reflection wave from the microwave resonator 102, the static magnetic field is swept by the electromagnets 104. When the ESR phenomenon is caused by the sweeping of the static magnetic field, absorption of the microwave by the sample 100 in the microwave resonator 102 is caused, a Q value of the microwave resonator 102 is changed to thereby cause reflection of the microwave, and a reflected microwave is extracted through the circulator 118. When the continuous wave ESR is executed, a path 122a is formed by a switch 120. The reflected microwave is supplied through the path 122a to a phase demodulator 126. Phase demodulation is executed by the phase demodulator 126 with respect to the reflected microwave and a reference signal which is sent through a phase shifter 124. With such a configuration, an absorption signal by the ESR phenomenon is detected. For example, an AC current of about 100 kHz generated at an oscillator 128 is supplied to the magnetic field modulation coil 106 so that a modulation magnetic field is superposed on the static magnetic field formed by the electromagnets 104, and an absorption signal modulated by 100 kHz is observed. The absorption signal is amplified by an amplifier, and phase demodulation is executed by a phase demodulator 130 (for example, a Phase Sensitive Detector (PSD)) using a reference signal supplied from the oscillator 128 (lock-in demodulation). A signal which is output from the phase demodulator 130 passes through a low-pass filter 131, and a continuous wave ESR spectrum signal 132 is obtained as a DC component.
Next, the pulse ESR will be explained. In the pulse ESR, the magnetic field modulation is not executed. A microwave generated by the microwave oscillator 108 is supplied through the switch 112 to a phase adjuster 134. The phase adjuster 134 is formed from, for example, a four-phase switch. The phase adjuster 134 has a function to output, for example, microwaves shifted in phase by 90° such as those at 0°, 90°, 180°, and 270°. With such a configuration, an arbitrary phase can be selected from the four phases. A microwave which is output from the phase adjuster 134 is supplied to a switch 136. With a switching operation (switching between ON and OFF states) by the switch 136, a microwave pulse is formed. The microwave pulse is amplified by an amplifier 138, and is supplied through the switch 114 and the circulator 118 to the microwave resonator 102. For the amplifier 138, for example, a power amplifier of an order of 1 kW (for example, a Travelling Wave Tube Amplifier (TWTA)) is used. The static magnetic field generated by the electromagnets 104 is fixed during one spin echo and during measurement of FID. For the spin echo and FID, integration processing is executed one or more times under a fixed static magnetic field. When ESR phenomenon is caused with the irradiation of the microwave pulse, a reflected microwave is extracted through the circulator 118. When the pulse ESR is executed, a path 122b is formed by the switch 120. In addition, during the measurement, a switch 140 is switched ON. The reflected microwave is supplied through the path 122b and the switch 140 to an amplifier 142. The reflected microwave which is amplified by the amplifier 142 is supplied to a phase demodulator 144. The phase demodulator 144 is a quadrature demodulator, and executes quadrature demodulation (orthogonal phase demodulation) using a reference signal sent through the phase shifter 124. With such a configuration, a real signal component 146 and an imaginary signal component 148 are obtained. With respect to these signal components, a process such as, for example, Fourier transform is applied. According to the pulse ESR, the spin echo and an FID signal are observed. For example, by irradiating a 180° pulse (π pulse) after irradiation of a 90° pulse (π/2 pulse), the spin echo is observed.
In the ESR apparatus shown in FIG. 7, a magnetization component (My component) orthogonal to the static magnetic field is detected. As other methods, methods of detecting physical quantities other than the My component are known. For example, longitudinally detected ESR (LOD-ESR), electrically detected magnetic resonance (EDMR), optically detected magnetic resonance (ODMR), and the like are known. These methods may be considered indirect ESR in a sense that physical quantities other than the My component are detected. FIG. 8 shows an ESR apparatus which realizes these methods.
First, the longitudinally detected ESR will be explained. In the longitudinally detected ESR, a change of an Mz component of electron spin (magnetization component parallel to the static magnetic field) is detected. For this purpose, a pickup coil 150 in which a wiring axis is placed in a direction parallel to the static magnetic field is placed near the sample 100. A microwave generated by the microwave oscillator 108 is attenuated by the attenuator 110 to a predetermined electric power, and is then supplied to a switch 156. Meanwhile, an oscillator 152 generates a reference signal having a modulation frequency. The reference signal is supplied through the switch 154 to a switch 156. The switch 156 repeats the ON and OFF states according to a modulation frequency. With such a configuration, the microwave is modulated according to the modulation frequency. The modulated microwave is supplied through the circulator 118 to the microwave resonator 102. When the ESR phenomenon is caused due to the sweeping of the static magnetic field, the Mz component of the electron spin changes, and an induced voltage is generated at the pickup coil 150. The induced voltage is amplified by an amplifier 158, and is supplied to a phase demodulator 160. The change of the induced voltage is synchronous with the modulation frequency. Therefore, lock-in demodulation is executed by the phase demodulator 160 (for example, PSD) using the reference signal supplied from the oscillator 152. A signal which is output from the phase demodulator 160 passes through a low-pass filter 161, and, with this process, a longitudinally detected ESR signal (LOD-ESR signal) 162 is obtained. With the use of the longitudinally detected ESR, it is also possible to observe a longitudinal relaxation time T1 (spin lattice relaxation time).
Next, the electrically detected magnetic resonance will be explained. The electrically detected magnetic resonance is a method in which a current (voltage) is applied to the sample 100 by a voltage supply and detection device 170, and a change of a current flowing in the sample 100 is detected. In this method, a microwave generated by the microwave oscillator 108 is modulated according to the modulation frequency by a switching operation of the switch 156. Alternatively, the microwave is not modulated, but the magnetic field is modulated. In this case, an AC current generated by the oscillator 152 is supplied through the switch 154 to the magnetic field modulation coil 106. The microwave is supplied through the circulator 118 to the microwave resonator 102, and, when the ESR phenomenon is caused as a result of the sweeping of the static magnetic field, the current flowing in the sample 100 changes. The current is detected by the voltage supply and detection device 170. A signal indicating an amount of this change is amplified, and is supplied to the phase demodulator 172. The change of the current is synchronous with the modulation frequency. Therefore, lock-in demodulation is executed by the phase demodulator 172 (for example, PSD) using the reference signal supplied from the oscillator 152. A signal which is output from the phase demodulator 172 passes through a low-pass filter 173, and, with this process, an EDMR signal 174 is obtained. With the use of the electrically detected magnetic resonance, it becomes possible to detect an electron spin resonance contributing to the change of the current. For example, a diode is used as the sample 100, and a defect of a semiconductor is observed.
Next, the optically detected magnetic resonance will be explained. The optically detected magnetic resonance is a method in which light is irradiated from a light source 180 onto a sample 100, and a change of an amount of light absorption by the sample 100 is detected. In this method, the microwave or the magnetic field is modulated, similar to the case of the electrically detected magnetic resonance. When the microwave is supplied through the circulator 118 to the microwave resonator 102, and the ESR phenomenon is caused as a result of the sweeping of the static magnetic field, an amount of absorption of light by the sample 100 changes. The light from the sample 100 is detected by an optical detector 182. A signal showing an amount of this change is supplied to a phase demodulator 184. The change of the amount of light absorption is synchronous to the modulation frequency. Therefore, lock-in demodulation is executed by the phase demodulator 184 (for example, PSD) using a reference signal supplied from the oscillator 152. A signal which is output from the phase demodulator 184 passes through a low-pass filter 185, and, with this process, an ODMR signal 186 is obtained.
In the ESR apparatus shown in FIG. 8, the continuous wave ESR is applied. Alternatively, the pulse ESR may be applied.
In addition, there is known a method in which the pulse ESR and the continuous wave ESR are combined, known as a hybrid ESR. In this measurement method, an electron spin resonance is excited by a microwave pulse, and the ESR signal is detected by lock-in demodulation. For example, in the pulsed LOD ESR described in a reference, A. Schweiger, R. Ernst, J., Magn. Reson. 77, 512 (1988), a pulse sequence that induces a change of magnetization in the Mz direction is executed, and a microwave pulse is supplied into the microwave resonator. For example, a 180° pulse (π pulse) is supplied according to a repetition frequency. A detection signal indicating an induced voltage from a pickup coil placed near the sample is lock-in demodulated using the repetition frequency of the pulse sequence. With such a configuration, a longitudinally detected ESR signal is obtained.
A reference, D. Lepine, Phys. Rev. B, Vol. 6, No. 2, 436 (1972), discloses a technique in the electrically detected magnetic resonance in which a change of intensity of the EDMR signal is recorded using the modulation frequency as a variable.
In a method in which the irradiation of the microwave pulse and the lock-in demodulation are combined such as the hybrid ESR described above, a problem may arise when the repetition frequency of the pulse sequence is changed. For example, there may be cases where it is desired to change the repetition frequency of the pulse sequence according to the sample or the measurement details. When a sample having a short relaxation time is to be measured, it may be desired to shorten the repetition frequency of the pulse sequence in order to shorten a wait time of measurement and to consequently improve measurement efficiency. On the other hand, when a sample having a long relaxation time is to be measured, it is necessary to increase the repetition period according to the duration of the relaxation time. The repetition frequency of the pulse sequence corresponds to a repetition frequency used in the lock-in demodulation. Because of this, when the repetition frequency of the pulse sequence is changed, the repetition frequency used in the lock-in demodulation must also be changed according to the change of the repetition frequency of the pulse sequence. However, when the repetition frequency used in the lock-in demodulation is changed, a frequency characteristic of a circuit must be changed. For example, in the longitudinally detected ESR, the resonance frequency of the pickup coil or the like must be changed every time the repetition frequency is changed. In addition, it becomes necessary to design the frequency characteristics of the resonance circuit and the amplifier to have a very wide range, or to replace the circuit itself.
An advantage of the present invention is that the lock-in demodulation is enabled without changing the frequency used in the lock-in demodulation even when the repetition frequency of the pulse sequence is changed in an electron spin resonance apparatus. An alternative advantage of the present invention is that the pulse sequence is repeated accurately.